\DeclareRobustCommand with active chars redefine control space (\ )

PhelypeOleinik commented 4 years ago

Brief outline of the bug

When \DeclareRobustCommand is used to define an active character, the internal definition is stored in the control space (\␣).

Minimal example showing the bug

\RequirePackage{latexbug}
\documentclass{article}
\begin{document}
\DeclareRobustCommand~{I'm a tilde!}
A control space: \ 
\end{document}

Proposed fix

The redefinition of the control space happens because the code does (assuming a printable \escapechar) \csname\expandafter\@gobble\string#1 \endcsname, then in the case of an active char the \@gobble consumes the entire char, so the \csname makes a space.

To avoid this we can ensure \escapechar=-1 and remove the \@gobble:

\RequirePackage{latexbug}
\documentclass{article}
\makeatletter
\def\declare@robustcommand#1{%
   \ifx#1\@undefined\else\ifx#1\relax\else
      \@latex@info{Redefining \string#1}%
   \fi\fi
   \edef\reserved@a{\string#1}%
   \def\reserved@b{#1}%
   \edef\reserved@b{\expandafter\strip@prefix\meaning\reserved@b}%
   \begingroup
     \escapechar=\m@ne
     \edef\reserved@a{%
   \endgroup
   \def\noexpand#1{%
      \ifx\reserved@a\reserved@b
         \noexpand\x@protect
         \noexpand#1%
      \fi
      \noexpand\protect
      \expandafter\noexpand\csname\string#1 \endcsname
   }%
   \let\noexpand\@ifdefinable\noexpand\@rc@ifdefinable
   \noexpand\new@command
     \expandafter\noexpand\csname\string#1 \endcsname}%
   \reserved@a
}
\makeatother
\begin{document}
\DeclareRobustCommand~{I'm still a tilde!}
A control space:\ .
And ~
\end{document}

Log file (required) and possibly PDF file

test.log

davidcarlisle commented 4 years ago

oops...

But just using escapechar=-1 isn't quite enough as then \~ and ~ both have the same internal form. I'll be offline most of today but

\documentclass{article}

\makeatletter

\def\declare@robustcommand#1{%
   \ifx#1\@undefined\else\ifx#1\relax\else
      \@latex@info{Redefining \string#1}%
   \fi\fi
   \edef\reserved@a{\string#1}%
   \def\reserved@b{#1}%
   \edef\reserved@b{\expandafter\strip@prefix\meaning\reserved@b}%
   \begingroup
     \escapechar=\m@ne
     \edef\reserved@a{%
   \endgroup
   \def\noexpand#1{%
      \ifx\reserved@a\reserved@b
         \noexpand\x@protect
         \noexpand#1%
      \fi
      \noexpand\protect
      \expandafter\noexpand\csname\string#1 \endcsname
   }%
   \let\noexpand\@ifdefinable\noexpand\@rc@ifdefinable
   \noexpand\new@command
     \expandafter\noexpand\csname\string#1 \endcsname}%
   \reserved@a
}

\makeatother

\begin{document}

\DeclareRobustCommand~{I'm still a tilde!}
\DeclareRobustCommand\~{But I'm  a tilde command!}

A control space:\ .

And ~

Or \~ j

\end{document}

PhelypeOleinik commented 4 years ago

Hm, yes, I forgot about \~... Well, with the current mechanism there's really no way to fit both ~ and \~, because \DeclareRobustCommand will define \~␣ for both since it strips the leading backslash, so their internal definition will always clash. We can make the internal definition for active chars <char><space><char>, so it won't clash with a command char (and since active chars didn't work anyway, I won't break anything ;-)


\RequirePackage{latexbug}
\documentclass{article}
\makeatletter
\def\declare@robustcommand#1{%
   \ifx#1\@undefined\else\ifx#1\relax\else
      \@latex@info{Redefining \string#1}%
   \fi\fi
   \edef\reserved@a{\string#1}%
   \def\reserved@b{#1}%
   \edef\reserved@b{\expandafter\strip@prefix\meaning\reserved@b}%
   \begingroup
     \escapechar=\m@ne
     \let\reserved@c\@empty
     \ifcat\noexpand~\noexpand#1%
       \edef\reserved@c{\string#1}%
     \fi
     \edef\reserved@a{%
   \endgroup
   \def\noexpand#1{%
      \ifx\reserved@a\reserved@b
         \noexpand\x@protect
         \noexpand#1%
      \fi
      \noexpand\protect
      \expandafter\noexpand\csname\string#1 \reserved@c\endcsname
   }%
   \let\noexpand\@ifdefinable\noexpand\@rc@ifdefinable
   \noexpand\new@command
     \expandafter\noexpand\csname\string#1 \reserved@c\endcsname}%
   \reserved@a
}
\makeatother
\begin{document}
\DeclareRobustCommand~{I'm still a tilde!}
\DeclareRobustCommand\~{But I'm  a tilde command!}

A control space:\ .

And ~

Or \~ j
\end{document}

stale[bot] commented 4 years ago

This issue has been automatically marked as stale because it has not had recent activity.

FrankMittelbach commented 2 years ago

I see 3 options:

ignore and the user has to live with the fallout
test for the arg being an active character and say, not allowed
test for the active character then use \csname romannumeral of-char-code @active\endcsname (or some such) define a robust command for that and \let the active char to it (may need a little more to preserve spacing)

hand-waving

stale[bot] commented 2 years ago

This issue has been automatically marked as stale because it has not had recent activity.

josephwright commented 1 month ago

Looking back, this is not supported behaviour - just needs documenting.

FrankMittelbach commented 1 month ago

test for the arg being an active character and say, not allowed

maybe we should do that as well, but perhaps overkill

latex3 / latex2e